Single photon sources
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Single photon sources are novel types of light sources distinct from coherent light sources (lasers) and thermal light sources (such as incandescent light bulbs and mercury-vapor lamps) that emit light as single particles or photons. The Heisenberg uncertainty principle dictates that a state with an exact photon number in a single frequency mode cannot exist because it must have infinite temporal extent. However, Fock states (or number states) can be studied for a system where the electric field amplitude is distributed over a narrow bandwidth. In this context, a single photon source gives rise to an effectively one-photon number state. Photons from an ideal single photon source exhibit quantum mechanical characteristics. These characteristics include photon antibunching, so that the time between two successive photons is never less than some minimum value.
Although the concept of a single photon was proposed by Planck as early as 1900. A true single photon source was not created in isolation until 1974. This was achieved by utilising a cascade transition within calcium atoms. Individual atoms emit two photons at different frequencies in the cascade transition and by spectrally filtering the light the observation of one photon can be used to 'herald' the other. The observation of these single photons was characterised by its anticorrelation on the two output ports of a beamsplitter in a similar manner to the famous Hanbury Brown and Twiss experiment of 1956.
Another single photon source came in 1977 which utilised the fluorescence from an attenuated beam of sodium atoms. A beam of sodium atoms was attenuated so that no more than one or two atoms contributed to the observed fluorescence radiation at any one time. In this way only single emitter were producing light and the observed fluorescence showed the characteristic antibunching. The isolation of idividual atoms continued with ion traps in the mid-1980s. A single ion could be held in a radio frequency Paul trap for an extended period of time (10min) thus acting as a single emitter of multiple single photons as in the experiments of Diedrich and Walther. At the same time the nonlinear process of parametric down conversion began to be utilised and from then until the present day it has become the workhorse of experiment requiring single photons.
Advances in microscopy led to the isolation of single molecules in the early 1990s, single pentacene molecules were detected in p-terphenyl crystals. Subsequently the single molecules have began to be utilised as single photon sources.
Within the 21st century vacancy centres in various materials, most notably nitrogen vacancy (NV) centers in diamond have also been utilised as a source of single photons.  These source along with molecules can use the strong confinement of light (mirrors, microresonators, optical fibres, waveguides, etc.)to enhance the emisstion of the NV centres. As well and NV centres and molecules, quantum dots (QDs) can emit single photons. And can be constructed from the same semiconductor materials as the light-confining structures.
Types of single photon sources
The generation of a single photon occurs when a source creates only one photon within its fluorescence lifetime after being optically or electrically excited. An ideal single-photon source has yet to be created. The requirements are that it should be on-demand, efficient, robust and easy to implement. Given that the main applications for a high-quality single photon source are quantum key distribution, quantum repeaters and quantum information science, the photons generated should also have a wavelength that would give low loss and attenuation when travelling through an optical fiber. Nowadays the most common sources of single photons are single molecules, diamond colour centres and quantum dots, with the last being widely studied with efforts from many research groups to realize quantum dots that fluoresce single photons at room temperature with photons in the low loss window of fiber-optic communication.
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